1. Field of the Invention
The present invention relates to an abnormality detection device for an air-fuel ratio sensor.
2. Description of the Related Art
In the past, there has been known an internal combustion engine designed to determine a fuel injection amount and control an air-fuel ratio based on an output signal from an air-fuel ratio sensor provided in an exhaust system. In such an internal combustion engine, when an abnormality occurs in the air-fuel ratio sensor, suitable control of the air-fuel ratio becomes difficult and, for example, deterioration of the emission or various other problems occur. Therefore, it is necessary to accurately detect an abnormality of the air-fuel ratio sensor by some method or another.
In this regard, for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-4580 describes a trouble diagnosis system for judging that trouble has occurred in an air-fuel ratio sensor, which gives a low output when the air-fuel ratio is lean and gives a high output when it is rich when the temperature is at least a predetermined activation temperature, when it is confirmed that it is in a high output state despite it being detected that the temperature of the air-fuel ratio sensor is lower than the activation temperature.
The system of the above publication, however, suffers from the problem that an abnormality can be detected only when the temperature of the air-fuel ratio sensor is less than the above activation temperature and an abnormality where the output of the air-fuel ratio sensor ends up becoming the lean side (that is, in this case, the low output side) cannot be detected. Further, it is difficult to detect an abnormality sufficiently accurately even by another abnormality detection system using an air-fuel ratio sensor proposed in the past. There is therefore a need for a novel abnormality detection system of an air-fuel ratio sensor.